Communication and guiding system



7, 1947. E. M. DELORAINE ETAL 1,017

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COMMUNICATION AND GUIDING SYSTEM Filed May 5, 1944 13 Sheets-Sheet 5 ATTOF/VEY y 27, 1947- E. M. DELORAINE ETAL 2,421, 17

COMMUNICATION AND GUIDING SYSTEM Filed May 5, 1944 13 Sheets-Sheet 4INVENTORS EDMO/VJ M aim/24w: BY 510A 1?. A04M5 ATTORNEY y 27, 1947- E.M. DELORAINE ETAL 2,421,017

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COMMUNICATION AND GUIDING SYSTEM Filed May 5, 1944 13 Sheets-Sheet 10syn/c 5455 WI] V5 6 JOURCE 418 E M00. M00- comma: saw/ 51? l cam mum]405 M00 4 5) P/l/ISE INVENTORS fiomo/vo M 0510/?4/11 5 BY PHI/L 1?.flD/IMJ A TT GENE Y May 27, 1947. E. M. DELORAINE ETAL 1,

COMMUNICATION AND GUIDING SYSTEM Filed May 5, 1944 13 Sheets-Sheet ll 76 0 2 4 m V i 2 6 .u 2 F F a W m 2 g r Ami MD M T. 2 mm wm m 2 w N Z 5MM mm 2 p z m o u M M74 s 7 1 0 w 4 fi) 4 m lulu I r m M 5 2w M a f 4 4MM rfl -I m E s 4 F: W H R 3 q M 4 2 H .w M W 4 m M l m ms a w a 4 a a ef f mu m 2% re 5 E01 2 J 4. 7 8

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COMMUNICATION AND GUIDING SYSTEM Filed May 5, 1944 15 Sheets-Sheet 12 AT TORNE Y 27, 1947. E. M. DELORAINE ETAL 2,421,017

COMMUNICATION AND GUIDING SYSTEM Filed May 5, 1944 13 Sheets-Sheet l3 E*Jno A T TGRJVE Y Patented May 27, 1947 FICE COMIWUNICATION AND GUIDINGSYSTEM Application May 5, 1944, Serial No. 534,284

37 Claims. 1

This invention relates to communication and guiding systems and moreparticularly to a system for communicating with a moving craft or othermobile communication unit and for guiding a craft or unit along aparticular route.

Systems have been previously proposed for simultaneously guiding a craftalong a given route and communicating with the craft from the beacon orguiding station. The systems of beacons for guiding crafts along a givencourse may comprise a series of radio beacons overlapping in space atpoints along the course with provisions, such as marker beacons,intermediate the radiation patterns of the separate beacons in theregion of the overlapping patterns warning when to change over forreception from a new beacon system. In one prior art arrangement, thereis provided a system in which the receiver on the craft is tuned inresponse to a change of frequency from one beacon to another so that theindicator will switch over without special attention of the pilot.However, in this type of beacon system, no provision is made to causethe craft indicatorto change its reception condition from one beacon toanother at substantially the same point regardless of the direction oftravel. Because of the inherent bias or sensitivity level needed toeffect the control for changing the receiver tuning, this type ofproposed beacon arrangement will cause the craft receiver to shift itsreception condition at difierent points along the course depending onthe direction of travel of the craft.

It is an object of our invention to provide a radio guiding beaconsystem in which a receiver carried by a craft following the beaconcourse from one beacon radiation pattern to another automaticallychanges its reception condition without special attention of the pilotand in which the change-over from reception of energy from one beacon tothe other will take place at substantially the same point, regardless ofthe direction of travel along the beacon path.

It is a further object of our invention to provide a radio beacon systemoperative in combination with a radio craft receiver which will provideon the craft a uniform directional indication when passing from onebeacon to another and in which the receiver is made to be selectivelyresponsive to the separate beacons without special attention from thepilot and will change in selective response along the course atsubstantially the same point, regardless of the direction of travel ofthe craft.

It is contemplated that, along a given course there may be a pluralityof different craft travelling and it is, therefore, desirable to be ableto communicate selectively with the difierent craft from a centralstation at any point along the beacon path. In a copending applicationof E. M. Deloraine, Serial No. 531,851, filed April 20, 1944, entitledRepeater link system, is disclosed a repeater system for multiplexcommunication between two remote points by way of a plurality ofdirective repeaters together with the provision of arrangements forbroadcasting or directively transmitting certain of the channels fromeach of the towers so that craft travelling along a line within range ofthese towers may communicate selectively with a base station or witheach other.

It is a further object of this invention to provide a combined multiplexcommunication system of the general type disclosed in the aforementionedpatent application, in combination with a beacon guiding system forguiding craft along a path defined by the line of radio towers.

It is a further object of our invention to provide a receiving circuitfor use on a craft which will selectively receive multiplexcommunication channels and simultaneously receive beacon indicatingsignals.

It is a still further object of our invention to provide a combinationcommunication system and beacon system in which a craft may follow alonga line of towers provided with radio beacon radiators operating atalternately difierent radio frequencies and in which there is providedaboard each craft means for simultaneously receiving communicationsignals and guiding beacon signals and for automatically rendering thereceiver operative selectively to the energy radiated from the alternatetowers.

It is a still further object of our invention to provide, in a system asoutlined above, means for assuring that the receiving equipment on boardthe craft changes from one alternate signal to the other substantiallyat the same point, regardless of direction of travel of the craft.

It is a still further object of our invention, to provide a system forcommunication with a craft and a guiding beacon therefore, in whichcommunication and guiding equipment is arranged at spaced points along acourse to be followed by a craft, and in which the equipment is renderedoperative in response to signals from the craft in the vicinity of oneof the stations, or to special control signals from a separate controlstation.

In accordance with a feature of our invention we may provide a multiplexcommunication system in which a plurality of channels are transmittedfrom a single terminal station over a plurality of successive towers. Ateach of the towers, or at selected ones of the towers, are arrangedradio beacons so aligned that energy from said beacons will slightlyoverlap the energy radiated from each succeeding beacon. Preferably theenergy from successive towers is radiated at slightly differentfrequencies to avoid danger of du licate reception. In the simplest formonly two difierent frequencies alternate along the course. However, morefrequencies may b used if desired, the frequency distributions to thedifferent towers preferably being repeated successively along thecourse. Also, at each of the towers or at selected ones of the towersare branched off one or more communicating channels for broadcasting tothe craft travelling along the course. The craft travelling along thecourse are each provided with receiving equipment for simultaneouslyreceiving the branched off channel signals as well as the guiding beaconsignal and fo segregating these signals to provide separate indications.Also, on the craft is provided some means responsive to the receivedenergy for rendering the receiving circuit responsive only to one of thesuccessive frequencies.

In order that the switching may take place when the craft is insubstantially the same area, regardless of the direction of travel ofthe craft. a separate marker beacon is provided adjacent the beginningof one of the beacon courses in the region of overlap of the twoadjacent beacons. This marker beacon preferably transmits a relativelynarrow fan shaped beam of energy upwardly, the energy beingcharacterized by the same frequency as the beacon whose radiationpattern overlaps the pattern of the next necessary beacon. Thus, in onedirection of travel, the response of the craft receiver will be changedimmediately after this marker beacon is passed due to the energyreceived from the beacon. In the other direction of travel, energy fromthe marker beacon will serve to switch the receiver to respond to energyfrom the next radio beacon. Accordingly, the switching over of the craftof these alternate frequencies will occur in substantially the samearea.

Furthermore, the craft is preferably provided with other communicationequipment permitting transmission of signals from the craft to thetowers along the course for communication with the base station or forcommunication between separate craft travelling along the course.

In the preferred form, the multiplex communication channels will eachconsist of a series of pulses separated properly in time so that theywill interleave one with another. These pulses may be modulated in anysuitable manner to provide pulse amplitude modulation or timedisplacement modulation Or any other desired type of modulation. Theparticular type of modulation circuit used is not a feature of thepresent invention.

Likewise, the communication between the tower and the plane and thesignals sent over the communication system may be of any desired form.Preferably, however, the beacon signals are of a relatively lowfrequency, for example, the normal 90-150 cycle tone frequencies or theknown form of A-N keyed signals. The communication channels arepreferably modulated pulse channels which may be readily separated fromthe guiding signals in the aircraft receiver by means of a clipperdevice so that segregation of the two channels may be accomplishedwithout heavy and complicated equipment.

According to a further feature of our invention, the successive towersand beacons are normally inoperative to broadcast to the craft or totransmit beacon guiding signals and at each tower or station is providedmeans responsive to signal from a craft in the vicinity thereof totransmit beacon signals, and/or to broadcast communication signals.Alternately, special signals from the base station may be transmitted tothe towers for the purpose of rendering the beacon and/or broadcastcommunication operative. Both these selective systems may beincorporated in a common system, if desired.

While the system of our invention is particularly useful for the guidingof aircraft, it is not to be considered as limited thereto. The beaconmay be used for guiding and/or establishing communication with any typeof mobile unit in the vicinity of the system.

A better understanding of our invention and the object and features maybe had from the particular description of certain embodiments thereof,made with reference to the accompanying drawing, in which:

Fig. 1 is a diagrammatic view in perspective i1- lustrating twoterminals of a multi-channel communication and guiding system togetherwith a chain of relay stations therebetween;

Fig. 2 is a block diagram of the west terminal station of a system suchas shown in Fig. 1;

Fig. 3 is a diagram partly in block of circuits illustrating therepeater tower equipment shown in Fig. 1;

Figs. 3A and 3B are block circuit diagrams of portions of the equipmentshown in Fig. 3;

Fig. 4 is a block circuit diagram of a craft receiver circuit for usewith the system shown in Figs. 2 and 3;

Fig. 4A is a curved used in explaining the operation of the selectorfeature of Fig. 4;

Fig. 5 is an elevational diagram of a beacon system in accordance withour invention illustrating the distribution of the various radiationfield patterns;

Fig. 6 is a diagrammatic sectional plan view along line 6--6 of theradio beacon field patterns illustrated in Fig. 5;

Fig. 7 is a set of curve diagrams illustrating operation of equipmentshown in Figs. 2, 3 and 4;

Fig. 8 is a wiring diagram of a time modulator of the type which may beused with our system;

Fig. 9 is a wiring diagram of selector and demodulator circuits usablewith a system in accordance with our invention;

Fig. 10 is a circuit diagram of a pulse width selector which may be usedin Figs. 2 and 3;

Fig. 11 is a graphical illustration used for illustrating the operationof the circuit of Fig. 10;

Fig. 12 is a block diagram of a modified form of equipment which may beused at a terminal station;

Fig. 13 is a block diagram of a tower equipment circuit arranged for usewith a terminal station, such as shown in Fig. 12;

Fig. 14 is a block diagram of an aircraft communication equipmentarranged for cooperation with the system of Fig. 13;

Fig. 15 is an elevational diagram of a modified beacon arrangementsimilar to that shown in Fig. 5;

Fig. 16 is a block circuit diagram illustrating the modified transmitterequipment used to produce the beacon arrangement of Fig. 15; and

Fig. 1'7 is a block circuit diagram of a receiver arrangement forresponse to signals such as transmitted from the circuit of Fig. 16.

Referring to Fig. 1, there are provided two terminals l and I2 (west andeast) interconnected by a chain of relay tower stations l3, l4 and I5.Of course, there may be many other repeater towers, only three beingshown for purposes of simplifying illustration. Each of the towers isprovided with a plurality of antennas 2|, 22 and 2|A, 22A, beingdirectional, as indicated. Energy is transmitted from west to east withdifferent characteristics between alternate towers, for example, atSlightly different radio frequencies Fl, F2, as shown in the drawing.

In the opposite direction, east to west, alternate frequencies, F3 andF4, are used. By the use of these different radio frequencies,interference of the signals by reason of overlapping of energy betweenthe repeater towers is minimized. Antennas 2|, 22 and 2|A, 22A arepreferably made sharply directional. At each of the towers are providedother antenna units 23 and 24 for transmitting and receivingcommunication signals to craft in the neighborhood of the variousstations. The transmitting antennas 23 may transmit energy at the samefrequencies Fl and F2, as used in the radio link system. However, theinbranching or incoming signals from the various craft are preferably ata different common frequency, herein shown as frequency F5.

Also mounted on each of the towers are a plurality of guiding beacontransmitting antennas 26 preferably provided with a directive reflectorarrangement 26A. These beacons transmitters may operate at the alternatefrequencies corresponding to the alternate frequencies Fl and F2,transmitted from the towers in the west-east direction. It should beclear that, if desired, the beacons may be arranged to cooperate withthe east-West channels instead. At each of the intermediate towers inthe zone of overlap of the directive radio beacon signals are providedseparate marker beacon transmitting antennas 25 operating at aparticular signal characteristic, for example, at the frequencycorresponding to the next preceding radio beacon pattern. Thearrangement of the guiding beacon and marker beacon system is providedfor assuring that craft travellin along a line defined by the beaconswill be enabled to switch from the reception of one of the frequenciesto the next adjacent one in substantially the same area, regardless ofthe direction of travel of the craft. This function will be explainedmore fully in detail later.

A better understanding of the system shown in Fig. 1 may be had from theparticular description of one embodiment of our invention as illustratedin Figs. 2, 3 and 4.

Referring to Fig. 2, the west terminal is shown to include a switchboard50 containing jack connections for outgoing and incoming channels l-Nand la-Na, respectively. The switchboard is of known character providedfor connectin telephone circuits or trunk lines to the proper channeljacks and also for connecting together incoming and outgoing channelsunder certain circumstances described hereinafter. Channel includes amodulator 5| coupled to terminal I. The modulator 5| and also all othersfrom modulator 52 to the modulator for channel N may be of any desiredtype but are preferably of the character for producing push-pull timedisplacement modulation of pulses. In order to effect this timemodulation, energy from a base wave source 60 is applied over individualphase shifters 62-41, etc., for channels 2 through N, respectively.Channel I need not have an individual phase shifter since the phase ofthe wave from source 60 may be applied directly without phase shift tothe modulator 5|. The phase shifters for the other channels are adjustedso that the individual trains of pulses for the separate channels willbe displaced in time to provide a single resultant train in the commonoutput 10.

Channel I has been chosen as the synchronizing and monitoring channeland, as such, is given a particular width Wl by shaper 58. The pulses ofthe other channels may be given a difierent width W2 in shaper 58a.

All the trains of pulses produced by the different channel modulatorsare combined, as indicated by the output connections at 10, and appliedto an R.--F. translator l4 and radiated by transmitting antenna 2|. TheR.-F. translator in this instance will provide a carrier wave offrequency Fl, as indicated in Fig. 1. This train of multiplex channelpulses is east bound, channel operating as a synchronizing andmonitoring channel for controlling the separation of channel pulses atthe receivers whether in a plane or at the east terminal l2.

In order that selected ones of the channels may be broadcast at terminall0, another modulator I6 is provided. This modulator is coupled to theselected channels to be broadcast transmitted at a wider angle forcommunication with craft. Separate couplers 7-8 are provided to preventthe other channels coupled in parallel to line 10 from also feeding tomodulator 16. These branched signals are then transmitted from antenna23 for a purpose of communication with craft as will be more readilyunderstood when the particular description of Fig. 4 is given.

Before discussing the equipment and operation of the relay stations, letit be assumed insofar as the west base is now concerned that a westbound train of channel pulses la, 2a, 3a Na is being received on antenna2lA, Fig. 2, and that the pulses thereof are detected into video form atreceiver 80. The video pulses of the incoming channels are applied to apulse width selector 8|. The video pulses passed by the selector 8| areapplied to a demodulator 84, the output of which is applied to thecorresponding switchboard terminal.

Energy from the pulse width selector is also applied to the deblockingselectors of the other receiving channels, the energy being appliedthrough suitable delay devices for proper timing. Referring to channel2a, for example, the deblocking pulse is applied to a delay device andthence to deblocking selector 92 where it controls the selection of thepulses of channel 2a, as received over line 9|, the selector excludingall other channel pulses. The channel pulses passed by the selector 92are applied to demodulator 94 whereby the time displacement of the videopulses is translated into an audio signal wave which, in turn, isapplied over line 95 to the proper jack on the switchboard 50. Thisselector control is the same for the additional channels 311 to Na andtherefore need not be illustrated or described in detail.

The return communication signals from the craft in the vicinity from thewest base terminal are received on antenna '24 and detected to audiofrequencies in a suitable detector arrangement l5. Detector 15 isprovided with a channel selector system for separating the receivedsignals in accordance with the selected channel and applying thesesignals to individual lines 11. A suitable selector for this purpose maybe made in accordance with the teachings hereinafter more fullyexplained in connection with Fig. 3B. The separate output lines areconnected over a cable 19 to the individual terminals Ia, 3a, 5a, Ia,etc., of switchboard 50.

In Fig. 3 is disclosed in more detail a typical block circuit diagram ofthe equipment provided with the repeater tower. Thus, as preferablyshown, there are provided four directive antenna arrangements 2|, 22,'2IA, 22A, coupled to the receiver-transmitter units 32, 3| and 43, 41,respectively, of the repeater circuit. From the antenna 22 are appliedpulses to the outbranching circuit I which serves to select the channelsit is desired to transmit from antenna 23 for reception on the separatecraft. These output signals are applied to antenna 23 over line IOI.Carrier source 36 supplies energy at frequency F2 to energize beaconmodulator 31 and the outbranching circuit I00. The energy received fromthe craft is applied from antenna 24 over line I03 to inbranchingcircuit I05 from whence it is supplied to antenna 22A together withsignals from west bound transmitter 41. Energy from oscillator 48A isapplied at frequency F3 to transmitter 41 and inbranching circuit I05.The inbranching circuit is timed by energy from receiver 43 as indicatedby line I05a. The marker beacon antennas 25 are energized by transmitterI02 operating at the frequency FI, the same as that received onreceiving antenna 22. For communication with aircraft antennas 23, 24and 25 are preferably mounted above a shielding sheet I04 mounted on theassociated tower. This sheet serves as an artificial ground for theseantennas.

At the tower, the guiding beacon antennas 26 are provided within anangular reflector 26A which may be mounted on the tower in the mannerillustrated in Fig. 1. The reflector arrangement is adjusted to causethe beacon antenna system to operate substantially unidirectionally.Furthermore, the horizontal portion of reflector 26A serves as anartificial ground to prevent much of the beacon energy from beingradiated earthward to cause troublesome reflections and the consequentfalse course indications. It should be understood that, if the spacingabove ground causes an unduly large number of lobes in the verticalplane, or too much trouble from ground reflection, the beacon antennamay be mounted directly adjacent the ground and the area therearoundsuitably cleared to prevent undesired refleeting action. The beaconantenna is illustrated only diagrammatically since the specific form ofbeacon used is not material to the principles of our invention. It ispreferable to make this beacon as simple as possible and of suchconstruction that little service is needed. Likewise, it is clear thatthe modulation may be produced either by keying the antennas alternatelyto shift the pattern or by modulating with different tone frequencies,such as 150 and 90 cycle tones commonly used in radio course beacons atthe present. Moreover, each beacon may be caused to transmit identifyingsignals if desired as well as the directive indications.

Turning now to Fig. 3A, a more particular explanation of theout-branching circuit I00 will be given. The energy from the antenna 22is applied to a detector I06 which detects the incoming waves to a videofrequency. Since detection will generally take place in receiver 3I ifthe pulses are to be transmitted at a different R. F. detector I06 maybe omitted and the signal taken from the output of this receiver. Thedetected pulse envelope is applied to pulse width selector I08 whichserves to select out the shaped pulses of channel I. These pulses arethen reshaped, if necessary, in shaper I I0 after which they are appliedover line I33 to modulator I34. The selected pulses from the output ofI08 are also branched over delay circuits I I3, H5, H1, I I9 and I2I andshapers H4, H6, H8, I20 and I22, respectively, and applied to channelselectors I24, I26, I28, I30 and I32, respectively with the channelSignals from detector I06 to select the pulses corresponding to channels3, 5, 1, 9 and I I, for example. These selected channels properly timedwith respect to channel I are then applied from the output of therespective selectors to line I33 from whence they are all applied tomodulator I34. The output of modulator I34 is connected by means of lineIOI to antenna 23, as shown in Fig. 3. Thus, the selected channels arebroadcast for selection and reception by craft within the field of thecommunication system.

The operation of the in-branching circuit I05 may best be understood byreference to Fig. 3B. In this figure, the input line I03 is shown asconnected to a detector I40 which serves to detect the received radiofrequency to video frequency producing in the output a plurality ofpulses corresponding to the channels received by antenna 24. In the formillustrated, each of these channels is represented by pulses ofdifferent widths. Accordingly, separate width selectors and demodulatorsare provided, as shown at I4I, I43, I45, I41, I49 and I5I. Just aboveselector MI is provided a base Wave generator I60 which is controlled byselector synchronzer I60a to produce a suitable base wave for timingthese received channels for insertion into gaps in the east-westmultiplexing channel of the system. Energy from this base wave generatoris applied over separate phase shifters I6I, I63, I65, I61, I69 and Illto the separate pulse modulators I42, I44, I46, I48, I50, I52,respectively. In these separate modulators are produced time modulationpulse trains corresponding to channels Ia, 3a, 5a, 1a, 9a, and I la,respectively. The output of all of these modulator circuits are combinedand applied to modulator I12 which serves to modulate the carrierfrequency F3. The resulting carrier frequency pulses are applied to theoutput of transmitter 41 for transmission together with the othermultiplex channels. It should be understood, likewise. that in thisparticular case, instead of providing R.-F. modulator I12 in thein-branching circuit itself, these separate pulses could be applied tothe input of the modulator in transmitter 41 used for the normalrepeated channels.

In Fig. 4 is illustrated a typical receiver circuit for use on a craft,such as an aircraft, for receiving the communication and guiding signalsfrom the tower of the system in accordance with our invention. In thiscircuit, there is provided aircraft receiving antenna I which receivesthe beacon signals and the communication signals transmitted from thetower by means of antennas 23 and 26. Antenna I80 is preferably coupledto a broad band receiver I8| tuned sufficiently broadly to receive bothcarrier frequencies FI and F2. In the output of receiver I 8I areprovided amplifying filter arrangements I82, I84 tuned to passfrequencies FI and F2, respectively. When the craft is in such aposition that energy at F2 predominates, the output from I84 may beapplied to an automatic volume control circuit I86 which serves to biasI82 so that substantially no energy at FI passes. Similarly, when thecraft passes to a region where Fl predominates A. V. C. circuit I88 inthe output of I82 biases circuit I84 so that substantially no energy atfrequency F2 is passed. Thus, as the craft travels along a coursedefined by the radio beacons of different frequencies, the receiver onthe craft is made to respond to one of these signals to the exclusion ofthe other. It should be understood that other signal characteristicsdifferent from frequency discriminations may be used to operate thereceiver switching if desired. The receiver energy is then detected indetector I81 to provide an envelope of the form shown in Fig. 4A. Thisenvelope has a low frequency wave component 2I5 corresponding to thebeacon signals and a pulse component 2I8 corresponding to the pulsecommunication signals. This composite wave from the output of detectorI81 is applied to a limiter I90 which serves to clip or limit theenvelope at a level 2I1 preferably substantially at twice the averageincoming carrier frequency so that the low frequency envelope 2I5 willbe passed but the high energy pulses will be limited in amplitude. Theoutput of limiter I90 is then applied to a selective filter I9I whichserves to remove the higher frequency pulse components still remainingand this output signal is then applied to a guide indicator I92 or toother visual or audible indicators, if desired.

The envelope wave is also passed to a clipper I93 which serves to removethe portion of the envelope below clipper level 2" to separate theincoming pulses from the lower frequency envelope and apply them to theselective circuit. These pulses are applied to a pulse width selectorI94 which serves to select the pulses of channel I and apply them to ademodulator circuit I95 from which they are applied to headphones orother audible signal source I96. These pulses of channel I arepreferably order channel impulses used for communicating to all of theplanes or other craft in the region and also may serve for selectinganother channel. Accordingly, the pulses from the output of pulse widthselector I94 are applied to an adjustable delay circuit I91 which servesto adjust the delay of the pulses in the output of this selector to aproper position for selection of other communication channels from amongthose received at I8I. These output pulses are applied to a deblockingselector I98 together with the clipped pulses from the output of clipperI93. In selector I98 the desired channel is selected by adjustment ofdelay circuit I91 and these selected pulses are applied to demodulatorcircuit I99 and from there to headphones 289.

For the return communication a microphone 20I is associated with theother channel receiver I96 and applies pulses to a pulse modulator 282.A control wave source 293 is provided for properly timing modulator 292.If desired, the control wave source may be synchronized with the outputpulses from adjustable delay network I91 or directly from the output ofselector I94. This control wave, together with the audio signals from281, produce in the output of modulator 222 pulses time displaced inaccordance with the signalling energy applied. These output pulses arethen shaped to a desired Width in pulse shaper circuit 284 and appliedto radio fre-.

quency modulator 2. A wave of carrier frequency F5 is also applied tomodulator '2 from a source 2I8- so that the combined signals may beradiated from antenna 2 I2 for reception on antennas 24 of the linkchannel. In addition to the control channel transmitter, is provided acommunication channel transmitter including a microphone 206 and a pulsegenerator and modulator 201. Output energy from control wave generator283 is applied over a phase shifter 205 to pulse generator 201 for thepurpose of properly timing these pulses to prevent overlapping of thepulses from modulator 282. The output pulses from modulator 281 areapplied to a pulse shaper 208 which serves to adjust the pulses in widthto correspond with any one of the channels Ia, 3a, 5a, 1a, 9a and Ila,which are used for communication between the plane or other craft andbase station. These pulses are then also applied to modulator 2I I fortransmission over antenna 2I2.

It is clear that the signals transmitted from the craft equipment willbe received in the tower equipment, such as shown in Fig. 3, anddemodulated and properly timed for application to the multiplex channelin proper timed relationship.

A better understanding of the beacon operation may be had from thefollowing description made with reference to Figs. 5 and 6 of thedrawing. As shown, each of the communication mediums for the twodirections comprises a series of repeaters having field patterns showndiagrammatically in Fig. 5 at 220, 22I, 222, 223 and 224 for theeastbound traffic and 240, 24I, 242, 243 and 244 for the westboundtraffic. In addition, the beacon signals transmitted from the successiontowers are shown at 225, 226, 221 and 228 of Fig. 5. This unidirectionalpattern is preferably made to have considerable energy component in thevertical plane so that aircraft may follow the course at many differentaltitude levels. Preferably, each of these guiding beacon patterns isproduced by two relatively narrow overlapping patterns 225, 225a, 226,226a, 221, 221a, 228 and 228a, as can be more clearly seen in Fig. 6.

It should also be realized that there may be a number of vertical maximaand minima points in the radiation lobes of the beacon patterns when theantennas are mounted high above the earth. For this reason, it isdesirable that the receiving switching system be not made too sensitivesince, at the fringes where the succeeding patterns tend to overlap oneanother, there might be caused a number of reversals and switches fromone pattern to the other before sufiicient domination of one of theradiation patterns was achieved. Because of the difficulty inmaintaining perfect alignment of the beacon stations, this switchingaction would be very confusing on the pilot, as it would tend to showhim suddenly bearing a considerable distance from the course. By makingthe receiver switching less sensitive so that considerable amplitudedifference is required for the switching action, this oscillation of thereceiver circuit between the two conditions at the points of overlap ofthe patterns may be avoided.

Because of the safety margin provided in sensitivity, an aircraft suchas shown at 222A proceeding from the west to east would have to crossthe boundary of the overlapping zone of radiation patterns 228 and 221to some point such as indicated by the dot 225A before the automaticswitching could take control. On the other hand, an ai fq fafil such as2223 travelling in the 0pposite direction, would have to pass out of theinfluence of radiation pattern 227, for example, and considerably intothe field of the next succeeding radiation pattern 226 to some point,such as shown at 225B before the switching over of the receiver would beeffective. This would result in aircraft travelling in oppositedirections switching over their control at different points along thepath. Such variation in switching points is generally not desirableparticularly in view of the communication with the base station mountedon similar type receivers. In order to assure that aircraft travellingin opposite directions will be switched over for reception of the nextsucceeding pattern in substantially the same zone or area, regardless ofthe direction of travel the additional marker beacons 230, 23!, 232 and233 may be provided. These beacons, as shown, are made to have patternsslanting forward in the direction of the repeater stations because ofthe particular characteristic shape of the guiding beacon radiationpatterns. Thus, craft 222A travelling west to east from one beacon toanother will still be switched at substantially the same point 225A.However, craft 222B travelling in the opposite direction will have itsreceiver switched by means of the energy in the marker beams 230 to 233,respectively, so that the switching over will take place insubstantially the same area as that of planes travelling west to east.In the case of plane 222B, for example, it will be switched over atabout point 225C. These marker beams are preferably made relativelynarrow in the line of travel of the craft and relatively widetransversely thereto as shown in Figs. and 6.

Fig. 6 may be considered as a plan view in cross section along lines 6-6of Fig. 5. This type of illustration is preferable since the markerbeams 230-233 are arranged at an an e d a accurate plan view wouldtherefore be less clear.

While we have shown preferably the use of unidirectional radio beacons,it is clear that bidirectional beacons may be used, if desired.Likewise, it is clear that the beacons need not be provided at eachrepeater station but may be provided at discretely placed intervals sothat adequate overlapping patterns are produced.

In order more clearly to understand the operation of the multiplexingsystem, as shown in the preceding figures, reference is made to Fig. 7.In this figure, curve a represents several trains of pulsescorresponding to the respective channels. It will be seen that thesucceeding channels I to l2 for example, are spaced in time so that theydo not overlap. Likewise, channel I is shown to be composed of pulses ofa different width from the other channels for the purpose of selectivelytiming the remote receiving equipment. The timing of the channels at thetransmitting end, for example, at the west base, may be accomplished bymeans of apparatus described more fully in connection with Fig. 8. Arectified wave 250 may be provided to produce channel I and by phaseshift a different rectified wave I used for production of the nextsucceeding channel. The progressive phase shift of the control wave maybe used successively to time the other channels of the system in asimilar manner.

In curve 0 the selected channels I, 3, 5, 1, 9 and H are illustrated inextended form with respect to the showing of curve (1. These pulses may,for example, represent the substantially evenly spaced pulses branchedout at the control towers for communication with the craft. Separationor selection of the channels may be obtained by delaying pulses ldifferent amounts so that they coincide in position with the pulses ofsuccessive channels, 3, 5, l, 9 and l I, as shown in curve d. If thesepulses are then combined and clipped at level 252, the channels may beseparated out from the entire group of channels. For example, if asingle channel is to be selected as shown in curve e pulse I will bedelayed to position it in time to coincide with channel 5 for theselection of this particular channel.

In curve f are shown pulses of different width which may be used on thecraft to indicate separate reply channels lb, 3b, 5b, 1b, 9b and llbwhich may correspond to the channels I, 3, 5, 1, 9 and II transmitted tothe craft. These separate channels are selected at the tower inaccordance with pulse width, as was explained more particularly inconnection with Fig. 3B. A more complete description of the pulse widthselector apparatus is made hereinafter in connection with Figs. 10 and11.

In order more clearly to disclose the structural examples of a pulsegenerator and modulator suitable for use in the transmitter circuits ofFigs. 2, 3 and 4, reference may be had to the circuit diagram of Fig. 8.Signal energy for channel I, Fig. 2, for example, is applied tomodulating circuit 5|, the output of which is coupled to the outgoingline 10. The input signals are applied to the grid circuit of couplingtube 300. The wave from tube 300 is applied over separate transformerwindings 304 and 305 to a mixing transformer arrangement 303.Simultaneously, energy from base wave generator 60 is applied to mixingtransformer 303 by means of coil 306. Secondary coils 301 and 308 arealso coupled to transformer 303 to extract from there the mixed signals.Coils 301 and 308 are coupled to a pair of triode amplifiers 309 and 3l0asymmetrically biased by batteries 3! I, 3l2 for operation in the mannerof an offset full-wave rectifier. The base wave of generator 60preferably has a frequency corresponding to the desired cadencefrequency T of the pulses to be produced.

Since tubes 309 and 3l0 operate in efiect as an offset full-waverectifier, an output wave of the form shown at 3|4a will be produced inthe ab sence of any input signals. This wave is applied to a pulseshaper 314 which clips and shapes the cusps of wave 3! to produce aplurality of unevenly spaced pulses (see dotted line pulses of pulsetrain 3| 5) having cadence frequency T. Upon application of energy overchannel connection 311, the effective bias of the push-pull circuit oftubes 309, 3| 0 is changed producing pulses having differentdisplacement in pushpull as indicated by curve 3| 3 and the solid linepulses of pulse train 3l5. This displacement represents applied energyof one sign, for example, positive. The displacement will also takeplace in the other direction for applied energy of different sign sothat the pulses will be displaced further from symmetry. Thus, as thesignals are applied to modulate the base wave, a time displacement ofthe pulses in the output of pulse shaper 3M is provided. While Fig. 8discloses a modulator circuit for block 5| of Fig. 2, it is clear thatthe same type of modulator may be used in all of the different pulsegenerator and modulator circuits of the system. For a furtherdescription of this type of modulator, reference may be made to thecopending application of E. Labin and D. D. Grieg, Serial No. 455,897,filed August 24, 1942.

A typical demodulator circuit for time modulated pulses is shown in Fig.9. This circuit corresponds to the combination of deblocking selector BIand demodulator arrangement 84 hereinbefore referred to in Fig. 2. Itshould be distinctly understood, however, that the demodulator 84 ofthis circuit may be used for demodulation in the other parts of thesystem.

According to the arrangement of Fig. 9, the train of pulses 32I is fedover line 83a to mixer tube 320 of the deblocking selector 92. At thesame time, deblocking pulses 323 from delay circuit 90 (Fig. 2) are fedto another separate grid of tube 320 over line 322. These deblockingpulses serve to produce, in conjunction with the selected incomingpulses timed to add hereto, an output series of pulses from tube 320.Assume, for example, that channel pulses of 32l are applied to theselector 9|, the delay line will operate to provide a series ofrectangular pulses, 323 timed in spaced relation according to the pulsesof channel 3. These combined pulses may have the form shown in curve 6,Fig. 7. Tube 320 is also biased to serve as a clipper so that only theboosted pulses of the selected channel appear in the output thereof. AD. C. restorer rectifier tube 330 may be provided across the input ofthe deblocking pulses 323 to assure that the incoming deblocking pulsesare of a proper level to work with the clipper circuit of tube 320 toleave only the desired output pulses above clipping level.

The output pulses from tube 320 have the same cadence frequency T as theoriginal modulated pulses 3l5 from modulator 303, Fig. 8. The pulses areapplied to the control grid of a demodulator tube 349 and cause tunedcircuit 3M connected to another grid of this tube to oscillate at thetuned frequency producing in the tube 349 a combined grid potential inthe form of a combination of the wave generated in 3H and the incomingpulses. Circuit 34l is preferably tuned to some harmonic of the cadencefrequency of the input pulses so that as the repetition rate is varied,due to the modulation signals, the output pulses will be raised todifferent levels depending upon their time displacement. Accordingly, inthe output of tube 340 will appear a modulation envelope of pulsescarrying signal modulations thereon. For a further understanding of theprinciples of this type of demodulator, reference may be had to thecopending application of D. D. Grieg, S. N. 459,959, filed September 28,1942.

A low-pass filter 345 is provided to remove from this demodulated signalenvelope the pulses of higher frequencies that define the signalenvelope.

While the pulse width selectors of Figs. 2 and 3 may comprise anycircuit arrangement capable of segregating a pulse of a given width frompulses of greater and/or lesser widths. we have shown in Fig. 10, forpurposes of illustration, a suitable circuit which, depending uponselection of circuit constants, is capable of distinguishing between twopulse widths difiering by as little as one hundredth of a microsecond.Such fine distinction, however, is not necessary except where selectionamong a very large number of difierent pulse widths are required betweenclose limits.

The circuit of Fig. 10 preferably includes a limit clipping stage 353 asan input coupler which limits all input pulses to substantially the sameamplitude. Should the input pulses be of a positive polarity asindicated by the pulses of curve Ila in Fig. 11, the coupled stage 350also serves to reverse the polarity as indicated by the pulses of curveI lb. This output pulse energy from stage 359 is applied through aresistor R to a shock excitable L-C circuit 355. Connected across thetunable circuit 355 is a vacuum tube 360, the cathode 36l of which isconnected to the input side of the circuit 355, while the anode 362 isconnected to the opposite side 363 of the tunable circuit. The side 363is also connected to a source of anode potential 364. The pulse energy,curve I lb, from the anode connection 354 is applied to the grid 365 ofthe tube 380 so as to block the conduction between the cathode 36l andthe anode 382 while pulse energy is applied to the circuit 355. Theundulations produced in the circuit 355 in response to pulse energy overanode connection 354 are taken oil through a connection 3'10 forapplication to a threshold clippin amplifier stage 315. The bias on thegrid 376 is controlled by adjustment of resistor 311. In the output 318of stage 215 is a pulse width shaper 389, the operation of which ishereinafter described.

Assume, for purposes of illustration, that the widths of the pulses ofcurves Ho and llb correspond, respectively, to channels I, 3, 5, l and9. as indicated by the width reference characters Wl, W3, W5, W1, andW9. Assume also that the circuit 355 is tuned for selection of pulsewidth W5. Curve I I0 represents the output of the circuit 355 when thecircuit 355 is tuned for selection of pulse width W5, illustrating thedifferent output undulations for the difierent pulse widths of curveIla. When the leading edge 38l of the pulse W5 is applied at negativepolarity, as indicated by curve llb, to the circuit 355, an initialundulation 382 is produced which is normally followed by undulations383, and so on in the form of a damped wave. When the circuit 355 istuned to a frequency, the period of which is exactly twice the width W5,the trailing edge 386 occurs where the initiated oscillatory energycrosses the zero axis from undulation 382 to undulation 383. Since thetrailing edge 386 shock excites the circuit in the same direction atthis point, an undulation 381 coinciding substantially with 383 producedthereby in the circuit 355 adds algebraically to the undulation 383 toproduce undulation 399. The next succeeding pairs of undulationsproduced by the leadin and trailing edges of pulse width W5 wouldnormally tend to produce a negative undulation which would continue as adamned wave. The damping tube 360, however, eliminates the trailingoscillations so that they do not interere wit the undulations producedby subsequent nulses a plied to the circuit 355.

A pulse width less than nulse width W5. such. for example. as pulsewidths WI and W3, will -ot pro ce maximum undu ations as great as theundulation 398 for the tuning adjustment corres onding to pulse widthW5. This is illustrated by the undulat ons 393 and 4 produced inresponse to the pulse widths W! and W3, res ectively. The reason forthis is readily a parent because the shock excitations nroduced bv theleading and trailing edges of the pulses of lesser width than W5 are, inpart. opposed to each other. as indicated y the broken lines associatedwith t e undulations 393 and 394. The undulations 395 and 396 producedin re ponse to the greater pulse widths W! and W9 are likewise smallerthan the undulation 39!! since here again the oscillations produced inresponse to the leading and trailing edges of the greater

